1. Field of the Invention
The present invention relates in general to an image-sensing device. More specifically, it relates to a MOS active pixel sensor unit with a shutter (photodiode).
2. Description of the Related Art
Charge coupled devices are well-known image sensing devices utilized in various applications. In addition to charge coupled devices, active pixel sensors are also applied as image sensors. MOS active pixel sensors work by using photodiodes in association with NMOS transistors.
FIG. 1 illustrates a circuit structure of a conventional MOS active pixel sensor unit. Referring to FIG. 1, the structure of the MOS active pixel sensor unit will be described as follows. The drain of a NMOS transistor T1 is coupled to a constant voltage source VB, the source of the NMOS transistor T1 is coupled to the cathode of a photodiode Dp, and the anode of the photodiode Dp is grounded. The drain of a NMOS transistor T2 is coupled to the cathode of the photodiode Dp, and the source of the NMOS transistor T2 is coupled to the gate of a NMOS transistor T3. The drain of the NMOS transistor T3 is coupled to the constant voltage source VB, and the source of the NMOS transistor T3 is coupled to the drain of a NMOS transistor T4.
The MOS active pixel sensor unit depicted in FIG. 1 senses the light intensity of an image and transforms it into an electronic signal via the photodiode Dp, and the electronic signal is outputted at the terminal readout (the source of the NMOS transistor T4). The operation of the MOS active pixel sensor unit is described as follows, with reference to the timing chart depicted in FIG. 2.
In time interval (1), signals S1 and S2 with high voltages "1" are inputted to the gates of the transistors T1 and T2, respectively, thereby turning on transistors T1 and T2. The constant voltage source VB charges the node A.
In time interval (2), signal S1 is switched to low voltage "0" such that the transistor T1 is turned off. Meanwhile, the photodiode senses the light intensity of an image and generates a light-induced current passing from node A through transistor T2 and photodiode Dp to a grounding reference. Consequently, the voltage at node A is discharged to a voltage of V.sub.A1. The duration of the time interval (2) is equivalent to the exposure time of the MOS active pixel sensor unit.
In time interval (3), both signals S1 and S2 are switched to low voltages "0" such that both transistors T1 and T2 are turned off, and thereby voltage V.sub.A1 is held at node A. Meanwhile, the signal S4 is switched to a high voltage "1" such that the transistor T4 is turned on, thereby obtaining a light-response voltage V1 by reading V.sub.A1 through transistors T3 and T4.
In time interval (4), both signals S2 and S4 are kept at high voltages "1", and the signal S1 is switched from a high voltage "1" to a low voltage "0". When the signal S1 is in a high voltage "1", the constant voltage source charges the node A. When the signal S1 is in a low voltage "0" (during the time interval exp), a light-reference voltage V2 is read from the transistor T4. Since the time interval exp is short, the exposure time of the MOS active pixel sensor unit is short enough such that the voltage drop at node A discharged by the light-induced current can be ignored. Consequently, the light-reference voltage V2, which is read during the time period exp, is equivalent to the voltage at the node A in the case of no light-induced current flowing through the photodiode Dp. The difference between the light-response voltage and the light-reference voltage is directly proportional to the light intensity.
FIG. 3 illustrates a circuit structure of an image-sensing device with resolution of 640.times.480 pixels, wherein the notation P represents a MOS active pixel sensor unit. 480 sensor units (P) share an output buffer and a load. All (640.times.480) active pixel sensor units of the image-sensing device sense images and store image data at the same time. Then, the data stored in the image-sensing device are read line by line (one line has 640 active pixel sensors).
The light-induced current generated by the photodiode Dp will change the voltage at node A into the voltage V.sub.A1 in the exposure process during time interval (2). In time interval (3), the voltage V.sub.A1 at node A is held by capacitors before it is read out. In FIG. 4, the parasitic capacitor Cs formed between the n-type diffusion drain of the transistor T2 and a p-type substrate, and the parasitic capacitor formed at the gate of the transistor T3, work together to store the voltage V.sub.A1 at node A.
However, the total capacitance at node A usually is too small to store large charge. Therefore, the voltage at node A can not be kept for a long time and is discharged quickly. After storing image data in all sensor units, the image data are read line by line as described above. If the voltage at node A in each of the unit sensor units can not be held for a long time, the image data stored in some sensor units could vanish before they are read. Therefore, the ability to reliably access the data stored in the image-sensing device is degraded if the voltage V.sub.A1 at node A can not be kept for a longer time.
Furthermore, the junction leakage in transistor T2 and the channel leakage due to the DIBL effect (Drain Induced Barrier Lowering effect) in transistor T2 will disturb and change the voltage V.sub.A1 at node A. To overcome this issue, the channel length of the transistor T2 is increased, however, the performance of the transistor T2 will degrade due to the clock feed-through effect.